1. Field of the Invention
This invention is in the field of cryogenic and/or refrigeration apparatus.
2. Description of the Prior Art
In a cryogenic apparatus in which cryogenic fluids are cooled to develop refrigeration at cryogenic temperatures, it is necessary to pass the cryogenic fluid through several heat exchange systems of increasingly lower temperatures. For example, in a cryogenic apparatus to liquify helium, it is often necessary to pass gaseous helium through as many as three heat exchangers followed by rapid expansion of the high pressure cold helium gas to liquify it. The temperature involved in such systems can reach values as low as a few tenths of a degree Kelvin. Because of this, it is essential that the helium gas be as free as possible of any gaseous contaminants. The presence of such contaminants would very rapidly plug the heat exchange passages because they freeze out as solids at the low temperatures involved, thus disabling the cryogenic apparatus. Even in closed systems, the helium gas can pick up gaseous contaminants including carbon dioxide, air (and thus, oxygen, nitrogen and argon), hydrogen and water vapor. When helium is withdrawn and replaced with a make-up stream, the amount of impurities in the make-up stream tends to be significantly elevated compared to the amount present in the pure feed stream.
Such gaseous impurities in the cryogenic fluid streams are commonly removed by passing the gas to be purified through a bed of adsorbent, such as a bed of activated charcoal in an adsorber unit. Nevertheless, when make-up streams are involved, it has invariably been the practice to combine the relatively highly contaminated, low-volume make-up stream with the relatively pure, high-volume, pure feed stream prior to passing it through the adsorber unit. Whereas this method does reduce the contaminants in the combined fluid stream, it is an inefficient removal process and it does not reduce the levels of contaminant to levels which are acceptable in many applications.